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Chapter 3: Varieties of analogue studies
Chapter 3: Varieties of analogue studies In essence, a natural analogue study can be any form of investigation of any relevant system, provided that it results in qualitative or quantitative information which can be used to support and build confidence in geological disposal. This may mean that an investigation provides data which are directly applicable to performance assessment (for the development of models or the provision of parameter values) or, alternatively, it may provide illustrations of concepts or processes that allow non-technical demonstrations of safety to be made. On this basis, two important guidelines for selecting analogue studies are focussed on the end-user of the analogue information: the output of the study needs to be defined clearly, together with the intended use of the analogue information, and the end-user (performance assessor or information presenter) should be involved and informed at each stage of the study, including at the very beginning when the studied is first planned.
a study may stem from careful management and dissemination of data. For example, predictive model testing at its most rigorous should be done 'blind' to the data against which it is being tested, as discussed in Section 5.1. This requires tight planning and control of precisely which data need to be collected, and how these data are released to the modellers (Pate et al., 1994; Alexander et al., 1998). Whilst each repository design will require unique information to assist in building and presenting a safety case, there are a number of broad areas where information is required generically in the geological disposal field. Historically, analogue studies have tended to focus on only a small number of types of natural system and, thus, analogue studies can be categorised into a few broad groups which are representative of some major components of a repository system or feature of its evolution. These groups are: 9
natural geological and geochemical systems;
9
archaeological systems; and
9
sites of anthropogenic contamination.
For example, performance assessment modellers are keen to test their conceptual and mathematical
Although
approaches against well-defined test cases, and
convenient way of presenting the most obvious
the best way to define such cases is with the active
analogue systems which have the potential to
participation of the modeller when selecting
provide relevant information, it is important to
this
general
categorisation
is
a
analogue sites and when designing projects. When recognise that useful data may be obtained from using analogues in this way, much of the value of
many types of environment. The objective of the
53
The geological disposal of radioactive wastes and natural analogues
following discussion is, thus, briefly to describe
In the first case, potential weaknesses of the
the features of typical analogue systems and to discuss some of their limitations,
analogy with the waste radionuclide mostly relate to differences between the natural and repository
Within
environments, in terms of water chemistry and
many
analogue
studies
there
is
considerable emphasis on sites containing natural radionuclides or stable isotopes of elements of direct relevance to those in a repository. However, several studies have investigated the behaviour of other naturally occurring chemical species which
source
terms
etc.
However.
differences
in
concentration also need to be considered; for example, naturally occurring radionuclides can now be measured at extremely low concentrations (detection limits of about 10 s atoms) which are of
are believed to behave in an analogous manner to
no possible radiological significance and are far
chemical species in the waste. Consequently, as well as discussing the broad natural analogue
assessment. It is questionable if the behaviour of
below the levels considered in performance
3.1 Chemical analogues
plutonium, for example, at very low water concentrations is representative of its behaviour at more repository relevant concentrations. Hence, such observations need to be interpreted with care.
The elements of relevance for radioactive waste management are widely spread throughout the
When the analogy is extended to other, naturally occurring radionuclides of the waste element of
periodic table. However, not all of these elements (or their individual nuclides) occur in nature, Depending on the natural availability of a particular radionuclide, a natural analogue study has three options:
interest, differences due to the decay process must be added to the caveats above. This is not generally aproblem for very long-lived (or stable) isotopes but there are well-known examples of decay-induced processes which can give rise to geochemical partitioning of shorter-lived nuclides, such as recoil, radiolysis, hot atom effect. For very
types it is also worth considering this concept of
chemical analogues.
9
9
to examine the radionuclide directly when it occurs in nature, such as in the case of the natural decay series radionuclides (Ivanovich
short-lived nuclides with, say, a half-life of less than one year, the timescale of ingrowth and decay
and Harmon, 1992);
may be much shorter than that of the process to
to examine naturally occurring isotopes of the same element, when the waste nuclide does not occur in nature, such as in the case of stable caesium or bomb fall-out 137Cs as an analogue of waste ] 35Cs;
be studied (e.g. sorption, precipitation, matrix diffusion etc.) and, hence, the observed behaviour of a short-lived nuclide cannot be extrapolated to
to examine a different naturally occurring element which behaves in a chemically similar manner to the waste element; and to examine the end fission products of parent radioisotopes no longer present, e.g. at the Oklo natural fission reactors (see Box 4) where the behaviour of in situ generated plutonium and iodine can be interpreted with respect to
longer-lived isotopes of the same element. Probably the best known example of a decay induced processes is the case of 234U/238U disequilibrium
in
natural
waters,
which
is
discussed in detail by MacKenzie et al. (1 990a). When considering a naturally occurring element as an analogue for a different waste element, great care must be exercised. The concept of chemical analogue
elements
was
first
discussed
by
Chapman et al. (1984) who presented a list of
possible chemical analogues for some of the longthe location of decay products such as 235U lived radionuclides present in HLW. Experience and 129Xe"
54
Varieties of analogue studies
Figure 3.1: Periodic table of the elements. Some examples of chemical or isotopic analogues are illustrated: (a) natural 226Ra for waste 226Ra; (b) fallout 137Cs for waste 135Cs; (c) stable I for waste 1291; (d) Re as an analogue for Tc; (e) Eu, or any lanthanide, as an analogue for a trivalent actinide, e.g. Am; (f) Th as an analogue for Pu(IV); (g) U, under oxidising conditions, as an analogue for Pu(VI). since this time has shown that this and similar tabulations can be misleading, and may lead to over-interpretation of the extent of the analogy involved. The true extent to which one element can be considered a chemical analogue of another is very dependent on the system and process studied, and must be examined on a case by case basis. In the simplest case, chemical analogues can be selected from the well established chemical periodicities. For example, an alkali metal such as rubidium would be expected to behave in a similar manner to its neighbour caesium, whilst barium might be considered as an analogue of strontium, and bromine as an analogue of iodine etc. These chemical analogues are shown in Figure 3.1. For elements of simple chemistry, such assumptions may be quite easy to justify but great care must be taken for more complex cases, especially if different valencies
occur over the range of
Eh/pH conditions
encountered
in the natural
environment. The limitations of the analogy must always be borne in mind, even in the 'simple' cases. For example, bromine may be a good analogue for iodine when examining transport processes in deep groundwaters but can be significantly less satisfactory when examining near-surface environments, due to the tendency of iodine to be associated with microbial processes in soils and sediments, and its variable redox behaviour. A particular challenge is to select chemical analogues for the transuranic elements, because they often have complex chemistry and are not found in significant concentrations in most natural systems. Americium and curium are similar, and are not redox-sensitive in natural waters, being present only in the III oxidation state. Obvious chemical analogues of the actinides would be any of the lanthanides, which have similar chemistry,
55
The geological disposal of radioactive wastes and natural analogues
although curium and europium should generally be avoided because they may diverge from the III
the periodic table. Critical to the behaviour of technetium, however, is the transition from
state (to IV and II, respectively) causing a chemical partitioning which has been observed in some cases, such as at Polos de Caldas (MacKenzie et al., 1990a; Miekeley et al., 1990a).
cationic species under reducing conditions to the anionic pertechnetate under oxidising conditions. The redox conditions under which this transition occurs will differ for technetium and rhenium.
Plutonium has an extremely complex chemistry and may be found in natural waters (as anthropogenic contamination) in four oxidation states (111to VI), all of which may be present in measurable concentrations simultaneously. Under reducing conditions (e.g. in the presence of ferrous iron), plutonium is found predominantly in the III and IV oxidation states (e.g. Schweingruber, 1983). Some III-valent (lanthanides) or IV-valent (thorium or, possibly, zirconium or hafnium)
Finally, it should be emphasised that there is no general recipe for assessing the relevance of a particular chemical analogue; every system must be evaluated separately and the processes occurring in nature compared to those expected in the repository.
elements can be considered to have similar behaviour but this analogy must be regarded with caution. The use of Th(IV) as a chemical analogue for Pu(IV) is extensively discussed by Eisenbud et
Various natural geological and geochemical systems may be investigated as analogues, provided they are appropriate to the repository
al., (1984) in relation to the Morro do Ferro analogue site. Under oxidising conditions, the IV, V and Vl states of plutonium may all be important, the higher states especially so in the presence of high carbonate concentrations. In such a case, the closest similarity to the PuO~ and PuO2+ species
3.2
Natural geological and geochemical systems
system of interest. The natural systems that have attracted the most interest are uranium orebodies, naturally occurring high-pH systems and naturally occurring metals, glasses and bitumens.
3.2,1
Uranium orebodies
may be uranium (found predominantly as UO2+ Economic and subeconomic primary and under oxidising conditions) but the analogy is not secondary concentrations of uranium occur in very close. In a system of varying redox conmany different geological environments. Their ditions, uranium is probably the only reasonable principal interest as a natural analogue lies in the analogue of plutonium but, especially here, the mechanisms which have been responsible for their analogy should be considered as qualitative only. original deposition, and any subsequent Neptunium has slightly less complex chemistry, remobilisation of the natural series radionuclides. being found predominantly as Np(V) in oxidising These processes are shown graphically in Figure conditions and Np(IV)in reducing waters. For the 3.2 and are analogous to those which might be reducing case, thorium may be a good analogue, expected to occur in the near-field of a HLW or with protactinium as a possible analogue for oxidising conditions. Again, only uranium could
spent fuel repository. Secondary deposits and remobilised regions adjacent to orebodies are of
be considered as a qualitative analogue for a case most interest because they usually form
at
of varying redox conditions,
of
Technetium is not found in significant quantities in most natural systems. The chemistry of technetium is similar to rhenium but quite different from manganese, its other neighbour in
56
temperatures
which
are
representative
conditions in a repository (i.e. < 100~ One limitation of many orebodies is that a number of the better known sites are at relatively shallow depths, where high fluxes of oxygenated ground-
Varieties of analogue studies
waters
will
be
dominating both the current and recent transport processes. For processes where it is interesting to extrapolate to the low-flux, chemically reducing repository conditions expected in a repository, this requires very careful characterisation of the hydrochemical history of the site. In addition, orebodies which have been actively mined may be so perturbed that it becomes difficult to define the natural boundary conditions to the geochemical and hydrogeological systems. Nonetheless, useful information can be obtained from mines and quarries, indicated by
Figure 3.2: Diagrammatic view of a generic orebody showing the principal as physical and chemical processes which potentially might be investigated in the natural analogue studies in this type of geological environment.
work on redox fronts performed at the Osamu Utsumi mine as part of the Polos de Caldas study (see Box 14). However, the results from these studies have yet to be explicitly included in performance assessment. The main features of uranium orebodies of potential relevance as an analogue are" 9
the composition,
long-term
stability and
corrosion/dissolution behaviour of uraninite as an analogue to spent fuel;
9
the role of redox processes in mobilising and retarding radionuclides, including redox fronts and other geochemical discontinuities;
9
the speciation and solubility controls of radionuclides in groundwaters, including colloid formation and behaviour; the downstream retardation processes affecting remobilised radionuclides, including sorption phenomena on various surfaces and diffusion into the rock matrix porosity; and
57
The geological disposal of radioactive wastes and natural analogues
the
ability
to
disequilibria various
use
natural
decay
series
to estimate the longevity of
mobilisation
and
deposition
The main features in clay formations of potential relevance as an analogue are: 9
the relative roles of diffusion and advection and small-scale physical heterogeneities"
9
the estimation coefficients;
processes,
3.2.2
Geochemical discontinuities in clays
Clays may be used as either a backfill or buffer in a repository, or may form the host rock itself, as discussed in Chapter 2. Geochemical transport processes in clays are not well understood, owing largely to the complexity of the multiple coupled processes of solute
of
elemental
the movement of redox fronts,
diffusion
including
movement along fractures in clays; the thermal stability of clays and thermal effects on transport properties, in cases where igneous bodies have intruded into clays; and
and clay interactions which not only drive chemical migration but also control the movement of water and the development of the hydraulic properties of the clay medium. Where geochemical discontinuities occur in clay as a result of heterogeneity in a sedimentary sequence or due to contact with a compositionally different material, such as an igneous intrusion
or
the
burial of an archaeological
artefact,
they offer the opportunity to study small scale migration and other
processes.
These processes are shown graphically in Figure 3.3.
58
Figure 3.3: Diagrammatic view of a generic clay formation showing the principal physical and chemical processes which potentially might be investigated in natural analogue studies in this type of geological environment.
Varieties of analogue studies
the chemical complexing behaviour of radionuclides and other trace elements in pore fluids of different compositions, and in contact
9
the nature and stability of colloidal species formed in high pH waters, and at the interface between high pH waters and neutral waters.
with organic material in clays.
3.2.4 3.2.3
Hyperalkaline environments
Hydrothermal systems
Natural hydrothermal systems have sometimes
Natural waters with unusually high alkalinities may be analogous to the porewaters found in
been advanced as being analogous to the nearfields of HLW or spent fuel repositories during the
cements and concretes. These offer the opportunity to study the hydrochemical behaviour
early period after disposal when temperatures around the waste are high. It has been suggested
of radionuclides and trace elements in conditions similar to those of some cementitious repository near-fields,
that they provide the opportunity to study geochemical transport processes in warm, nearfield fluids.
Hyperalkaline waters are relatively rare in nature and have their origin in uncommon systems such as complex rock/water interactions involving the alteration of ultramafic rocks (e.g. at the Oman analogue site) or thermally metamorphosed limestones and marls (e.g. at the Maqarin analogue site). To be reasonably representative of cementitious porewaters, the pH must be greater
Unfortunately, the analogy is generally weak, because repository host rock temperatures in most designs (with the possible exception of the proposed US repository at Yucca Mountain, see Section 2.3.1) generally never reach more than about 80 to 100~ which is lower than the temperature of most hydrothermal systems. In any event, most disposal concepts aim at complete containment of the wastes within the waste package during the early thermal peak. In addition, most natural hydrothermal systems have been flushed with much larger volumes of fluids than would occur around a repository, although the duration and depth of water circulation in the two systems might be comparable (Cathles and Shea, 1992).
than 12. The main features in environments of potential analogue are: 9
natural high-pH relevance as an
the solubilities and speciation of radionuclides, and other relevant elements in high pH conditions where solubility calculations suggest rates of mobilisation into solution will be very slow; the interaction of high pH fluids with surrounding rocks which may be analogous to near-field porewaters which migrate from a repository into the host rock; the nature and viability of microbially mediated geochemical processes at high pH, with respect to their potential significance for wasteform breakdown and near-field radionuclide mobilisation" and
Combined, these differences between the hydrothermal and repository systems mean that the supposed near-field rock-water interactions and mass transport analogy may be exaggerated both in terms of the scales and amounts of materials involved, and the nature and kinetics of the reactions observed. In addition, attempts to extract pertinent information for performance assessment from hydrothermally altered granites are usually made difficult by the superimposition of several hydrothermal events at various temperatures.
59
The geological disposal of radioactive wastes and natural analogues
Nonetheless, a few studies (e.g. M~nager et al., 1992a; Parneix, 1992; Shea, 1998) have been described which claim to have extracted some useful information from hydrothermal or fossil hydrothermal systems. Thus, it may be possible, although very difficult, to obtain usable information on elevated temperature near-field processes from these geological systems but the limitations of the analogy must be borne in mind, and the guideline to 'define the end-use' must be remembered. The main features in hydrothermal systems of potential relevance as an analogue are: alteration of physical properties (e.g. pore and fracture enlargement or blocking) and chemical properties (e.g. fracture and pore surface mineralogy)of near-field rocks which could occur during a thermal transient and which may affect transport and sorption of radionuclides released at a later time; the solubility and speciation of radionuclides, and relevant trace elements in geothermal fluids where hydrothermal processes have affected an existing orebody or region of rock with elevated concentrations of relevant elements; and elemental and isotopic matrix diffusion profiles associated with veins containing hydrothermally transported and deposited minerals containing relevant trace elements, from which apparent diffusivity values can be determined, These analogue applications are only really useful to high temperature repository concepts or to scenarios where early canister failure during the initial thermal peak are considered. Otherwise, these applications appear tenuous for most repository designs and normal evolution scenarios, which perhaps explains the lack of interest by performance assessment modellers in using information derived from hydrothermal systems.
60
3.2.5
Natural occurrences of repository materials
This category includes a wide range of naturally occurring materials analogous to wastes or other repository components. These are generally found as isolated and often uncommon occurrences in nature. Typical examples are glassy igneous rocks, tektites, natural bitumens, iron meteorites and native metals. These materials are appealing analogues, but frequently they are compositionally inappropriate when compared to repository materials, to an extent which may make their properties and behaviour fundamentally different. Some good analogues can, however, be found, although careful evaluation is essential if data are to be used in performance assessment. As with archaeological analogues (see below), a critical aspect is the physical and chemical nature of the environment in which they are found, and its relevance to that of a repository. The range of studies which can be envisaged is, thus, very dependent on the exact characteristics of the material and of the host system. However, natural materials also have considerable potential for illustrating the basic concepts of repository design and performance for non-technical audiences and, in this case, the demands on the analogue may be relaxed somewhat. The types of studies that have been undertaken include: 9
natural volcanic glass corrosion as an analogue for borosilicate glass wasteform dissolution" natural metal corrosion and pitting as an analogue for metal HLW and spent fuel canister corrosion; decomposition of naturally occurring bitumen as an analogue for the degradation of the bitumen L/ILW immobilisation matrix; and stability of naturally occurring cement minerals as an analogue for the long-term behaviour of cement in a L/ILW repository.
Varieties of analogue studies
3.3 Archaeological analogues The use of archaeological analogues has grown in importance over the last few years as their potential for performance assessment support and, particularly, for providing illustrations for non-technical demonstrations of safety has been more widely acknowledged (Miller and Chapman, 1995; Miller, 1996b). The progressive decay of man-made artefacts can provide a direct analogy to the long-term behaviour of repository materials and can be studied at carefully chosen archaeological sites (e.g. Figure 3.4). The types of artefacts useful as analogues ranges from jewellery to buildings. The time period of study is, naturally, constrained to a few hundreds or thousands of years at most for the majority of relevant materials. However, although shorter in comparison to geological
analogue systems, the timescales applicable to archaeological analogues may be more tightly constrained. Although the term 'archaeological' is often used, some man-made systems which are only a few decades old also yield useful information on the rates of processes relevant to the early life of repositories. In some cases, the terms anthropogenic analogue or industrial
analogue are used instead. Due to the fact that archaeological materials and the environments in which they occur can be quite dissimilar to a repository situation, it is important that care is taken when selecting archaeological systems for investigation. Not all well-preserved, old artefacts can provide quantitative analogue information for input to performance assessment. However, archaeological analogues are also very important as providers of non-technical information and as illustrations of the basic
Figure 3.4. Archaeological artefacts can be used as analogues for the long-term behaviour of engineered barrier materials. In this case, an excavation of a Roman settlement in Colchester, England provided a range of iron and bronze artefacts (rods and rings which might have been used for divination). Qualitatively, this shows the much slower corrosion of iron over bronze (copper alloy). If the chemistry of the metals and the burial environment can be shown to be similar to the repository, then quantitative corrosion rates for materials may possibly be obtained. From Crummy (1997).
61
The geological disposal of radioactive wastes and natural analogues
concepts of repository design and performance for
However, this type of investigation is potentially
non-technical audiences. This is partly due to the fact that archaeological systems can sometimes be
prone to bias if focussed on artefacts obtained from museum collections, because museums will
associated directly with materials, artefacts and time periods with which people are familiar.
(naturally) tend to house the best preserved artefacts. Corrosion rates based solely on
To aid the selection of suitable archaeological analogues for both quantitative and qualitative uses, Miller and Chapman (1995) drew-up a list of basic parameters which should be known about
archaeological
material could, thus,
conservative. This sample bias problem is likely to be less important if artefacts are collected in situ, rather than from a museum, for then it would be possible to see artefacts
the artefact, these include:
be non-
in all possible
degradation states for that environment. Analogue 9
the age of the artefact;
9
the composition of the material;
9
the deposition and preservation history; the physico-chemical environment in which the artefact was preserved;
studies based on archaeological artefacts must be considered carefully to determine if this type of bias has occurred. Nonetheless, considering all the caveats on the suitability of archaeological artefacts, the range of materials and issues for investigation is potentially wide and includes:
any events or environmental changes which have affected the artefact when in situ; and
corrosion of metallic or cementitious objects analogous to waste containers or waste matrices;
the composition of the surrounding soil or sediment. A key issue is that the burial environment in which an artefact is preserved is as important as the artefact's composition for defining its suitability as an analogue. However, characterising the burial environment can be difficult, especially determining if the environment has been constant over time. Most artefacts come from the surface or from shallow depths, often in conditions which have been more chemically aggressive than those expected in a repository, e.g. in terms of chemical fluxes and redox environment. Clearly, if the burial
degradation of glasses and cementitious or bituminous materials analogous to wasteforms; 9
long-term evolution of the physico-chemical properties of cements and other building materials analogous to repository structures;
9
decay and breakdown products of organic materials and complexation with trace elements, analogous to waste degradation; and chemical interaction of buried objects with surrounding rocks and soils which may be analogous to near-field processes.
environment has been highly chemically active, most
artefacts
will
have
been
significantly
degraded. The general approach to archaeological materials is, therefore, one of trying to estimate rates and
mechanisms
of degradation
as a
function of the chemistry of the environment of preservation, and to extrapolate or interpolate the data from one or more sites or objects to the chemical conditions expected in a repository.
62
3.4 Sites of anthropogenic contamination There are many thousands of sites around the world
where carelessly disposed wastes and
spillages
have resulted
contaminants
through
in the
migration
subsurface
of
materials.
Several of these contaminated sites have been
Varieties of analogue studies
studied as anthropogenic analogues. For example,
may occur in a repository far-field. This is
investigations have been made of:
important because, if contaminant concentrations
the migration at the geosphere-biosphere interface of radionuclides released from the Chernobyl nuclear power plant explosion; the migration
through volcanic rocks of
radionuclides released from underground nuclear explosions at the nuclear weapons test site in Nevada; and
are too high, then they may display solubility limited transport behaviour. This would be in contrast to the repository system in which many elements will be released in such dilute concentrations that they would be expected to remain below solubility limits, especially in the farfield.
the migration through rocks and sediments of radionuclides leaked from liquid radioactive waste storage tanks and shallow LLW disposal
If suitable anthropogenic sites can be found with a demonstrable analogue potential, then it may be possible to investigate migration and retardation of transuranic elements in the subsurface
trenches,
environment and, hence, provide a means to support laboratory data for these non-natural
Several other similar contamination systems could be imagined as potential analogues. For example, the injection facilities for the disposal of liquid radioactive wastes in Russia, described in Section
nuclides. Nonetheless, particular consideration and care would need to be given to the extrapolation of data from any anthropogenic
2.3, are planned to be investigated in further detail
contamination site to the repository system.
to understand how these wastes have behaved and migrated underground since injection, as an analogue of radionuclide migration away from a repository. The problem with many of these anthropogenic contamination sites is that the environments are grossly unrepresentative of either the near or farfields of any deep geological repository concept. In some cases the physical systems are greatly perturbed (such as around sites of nuclear weapons tests) or the chemistry is too far removed from repository conditions to warrant being considered as an analogue (as may be the case at the some liquid waste injection sites). As a result, the great majority of sites of anthropogenic contamination have little or no analogue relevance for a radioactive waste repository. Nonetheless, there may be a few anthropogenic contamination sites from where useful analogue information could be obtained. Of particular interest would be locations where radioactive materials
have
migrated
into
otherwise
undisturbed rocks at concentrations sufficiently dilute to be representative of the releases which
63
a study may stem from careful management and dissemination of data. For example, predictive model testing at its most rigorous should be done 'blind' to the data against which it is being tested, as discussed in Section 5.1. This requires tight planning and control of precisely which data need to be collected, and how these data are released to the modellers (Pate et al., 1994; Alexander et al., 1998). Whilst each repository design will require unique information to assist in building and presenting a safety case, there are a number of broad areas where information is required generically in the geological disposal field. Historically, analogue studies have tended to focus on only a small number of types of natural system and, thus, analogue studies can be categorised into a few broad groups which are representative of some major components of a repository system or feature of its evolution. These groups are: 9
natural geological and geochemical systems;
9
archaeological systems; and
9
sites of anthropogenic contamination.
For example, performance assessment modellers are keen to test their conceptual and mathematical
Although
approaches against well-defined test cases, and
convenient way of presenting the most obvious
the best way to define such cases is with the active
analogue systems which have the potential to
participation of the modeller when selecting
provide relevant information, it is important to
this
general
categorisation
is
a
analogue sites and when designing projects. When recognise that useful data may be obtained from using analogues in this way, much of the value of
many types of environment. The objective of the
53
The geological disposal of radioactive wastes and natural analogues
following discussion is, thus, briefly to describe
In the first case, potential weaknesses of the
the features of typical analogue systems and to discuss some of their limitations,
analogy with the waste radionuclide mostly relate to differences between the natural and repository
Within
environments, in terms of water chemistry and
many
analogue
studies
there
is
considerable emphasis on sites containing natural radionuclides or stable isotopes of elements of direct relevance to those in a repository. However, several studies have investigated the behaviour of other naturally occurring chemical species which
source
terms
etc.
However.
differences
in
concentration also need to be considered; for example, naturally occurring radionuclides can now be measured at extremely low concentrations (detection limits of about 10 s atoms) which are of
are believed to behave in an analogous manner to
no possible radiological significance and are far
chemical species in the waste. Consequently, as well as discussing the broad natural analogue
assessment. It is questionable if the behaviour of
below the levels considered in performance
3.1 Chemical analogues
plutonium, for example, at very low water concentrations is representative of its behaviour at more repository relevant concentrations. Hence, such observations need to be interpreted with care.
The elements of relevance for radioactive waste management are widely spread throughout the
When the analogy is extended to other, naturally occurring radionuclides of the waste element of
periodic table. However, not all of these elements (or their individual nuclides) occur in nature, Depending on the natural availability of a particular radionuclide, a natural analogue study has three options:
interest, differences due to the decay process must be added to the caveats above. This is not generally aproblem for very long-lived (or stable) isotopes but there are well-known examples of decay-induced processes which can give rise to geochemical partitioning of shorter-lived nuclides, such as recoil, radiolysis, hot atom effect. For very
types it is also worth considering this concept of
chemical analogues.
9
9
to examine the radionuclide directly when it occurs in nature, such as in the case of the natural decay series radionuclides (Ivanovich
short-lived nuclides with, say, a half-life of less than one year, the timescale of ingrowth and decay
and Harmon, 1992);
may be much shorter than that of the process to
to examine naturally occurring isotopes of the same element, when the waste nuclide does not occur in nature, such as in the case of stable caesium or bomb fall-out 137Cs as an analogue of waste ] 35Cs;
be studied (e.g. sorption, precipitation, matrix diffusion etc.) and, hence, the observed behaviour of a short-lived nuclide cannot be extrapolated to
to examine a different naturally occurring element which behaves in a chemically similar manner to the waste element; and to examine the end fission products of parent radioisotopes no longer present, e.g. at the Oklo natural fission reactors (see Box 4) where the behaviour of in situ generated plutonium and iodine can be interpreted with respect to
longer-lived isotopes of the same element. Probably the best known example of a decay induced processes is the case of 234U/238U disequilibrium
in
natural
waters,
which
is
discussed in detail by MacKenzie et al. (1 990a). When considering a naturally occurring element as an analogue for a different waste element, great care must be exercised. The concept of chemical analogue
elements
was
first
discussed
by
Chapman et al. (1984) who presented a list of
possible chemical analogues for some of the longthe location of decay products such as 235U lived radionuclides present in HLW. Experience and 129Xe"
54
Varieties of analogue studies
Figure 3.1: Periodic table of the elements. Some examples of chemical or isotopic analogues are illustrated: (a) natural 226Ra for waste 226Ra; (b) fallout 137Cs for waste 135Cs; (c) stable I for waste 1291; (d) Re as an analogue for Tc; (e) Eu, or any lanthanide, as an analogue for a trivalent actinide, e.g. Am; (f) Th as an analogue for Pu(IV); (g) U, under oxidising conditions, as an analogue for Pu(VI). since this time has shown that this and similar tabulations can be misleading, and may lead to over-interpretation of the extent of the analogy involved. The true extent to which one element can be considered a chemical analogue of another is very dependent on the system and process studied, and must be examined on a case by case basis. In the simplest case, chemical analogues can be selected from the well established chemical periodicities. For example, an alkali metal such as rubidium would be expected to behave in a similar manner to its neighbour caesium, whilst barium might be considered as an analogue of strontium, and bromine as an analogue of iodine etc. These chemical analogues are shown in Figure 3.1. For elements of simple chemistry, such assumptions may be quite easy to justify but great care must be taken for more complex cases, especially if different valencies
occur over the range of
Eh/pH conditions
encountered
in the natural
environment. The limitations of the analogy must always be borne in mind, even in the 'simple' cases. For example, bromine may be a good analogue for iodine when examining transport processes in deep groundwaters but can be significantly less satisfactory when examining near-surface environments, due to the tendency of iodine to be associated with microbial processes in soils and sediments, and its variable redox behaviour. A particular challenge is to select chemical analogues for the transuranic elements, because they often have complex chemistry and are not found in significant concentrations in most natural systems. Americium and curium are similar, and are not redox-sensitive in natural waters, being present only in the III oxidation state. Obvious chemical analogues of the actinides would be any of the lanthanides, which have similar chemistry,
55
The geological disposal of radioactive wastes and natural analogues
although curium and europium should generally be avoided because they may diverge from the III
the periodic table. Critical to the behaviour of technetium, however, is the transition from
state (to IV and II, respectively) causing a chemical partitioning which has been observed in some cases, such as at Polos de Caldas (MacKenzie et al., 1990a; Miekeley et al., 1990a).
cationic species under reducing conditions to the anionic pertechnetate under oxidising conditions. The redox conditions under which this transition occurs will differ for technetium and rhenium.
Plutonium has an extremely complex chemistry and may be found in natural waters (as anthropogenic contamination) in four oxidation states (111to VI), all of which may be present in measurable concentrations simultaneously. Under reducing conditions (e.g. in the presence of ferrous iron), plutonium is found predominantly in the III and IV oxidation states (e.g. Schweingruber, 1983). Some III-valent (lanthanides) or IV-valent (thorium or, possibly, zirconium or hafnium)
Finally, it should be emphasised that there is no general recipe for assessing the relevance of a particular chemical analogue; every system must be evaluated separately and the processes occurring in nature compared to those expected in the repository.
elements can be considered to have similar behaviour but this analogy must be regarded with caution. The use of Th(IV) as a chemical analogue for Pu(IV) is extensively discussed by Eisenbud et
Various natural geological and geochemical systems may be investigated as analogues, provided they are appropriate to the repository
al., (1984) in relation to the Morro do Ferro analogue site. Under oxidising conditions, the IV, V and Vl states of plutonium may all be important, the higher states especially so in the presence of high carbonate concentrations. In such a case, the closest similarity to the PuO~ and PuO2+ species
3.2
Natural geological and geochemical systems
system of interest. The natural systems that have attracted the most interest are uranium orebodies, naturally occurring high-pH systems and naturally occurring metals, glasses and bitumens.
3.2,1
Uranium orebodies
may be uranium (found predominantly as UO2+ Economic and subeconomic primary and under oxidising conditions) but the analogy is not secondary concentrations of uranium occur in very close. In a system of varying redox conmany different geological environments. Their ditions, uranium is probably the only reasonable principal interest as a natural analogue lies in the analogue of plutonium but, especially here, the mechanisms which have been responsible for their analogy should be considered as qualitative only. original deposition, and any subsequent Neptunium has slightly less complex chemistry, remobilisation of the natural series radionuclides. being found predominantly as Np(V) in oxidising These processes are shown graphically in Figure conditions and Np(IV)in reducing waters. For the 3.2 and are analogous to those which might be reducing case, thorium may be a good analogue, expected to occur in the near-field of a HLW or with protactinium as a possible analogue for oxidising conditions. Again, only uranium could
spent fuel repository. Secondary deposits and remobilised regions adjacent to orebodies are of
be considered as a qualitative analogue for a case most interest because they usually form
at
of varying redox conditions,
of
Technetium is not found in significant quantities in most natural systems. The chemistry of technetium is similar to rhenium but quite different from manganese, its other neighbour in
56
temperatures
which
are
representative
conditions in a repository (i.e. < 100~ One limitation of many orebodies is that a number of the better known sites are at relatively shallow depths, where high fluxes of oxygenated ground-
Varieties of analogue studies
waters
will
be
dominating both the current and recent transport processes. For processes where it is interesting to extrapolate to the low-flux, chemically reducing repository conditions expected in a repository, this requires very careful characterisation of the hydrochemical history of the site. In addition, orebodies which have been actively mined may be so perturbed that it becomes difficult to define the natural boundary conditions to the geochemical and hydrogeological systems. Nonetheless, useful information can be obtained from mines and quarries, indicated by
Figure 3.2: Diagrammatic view of a generic orebody showing the principal as physical and chemical processes which potentially might be investigated in the natural analogue studies in this type of geological environment.
work on redox fronts performed at the Osamu Utsumi mine as part of the Polos de Caldas study (see Box 14). However, the results from these studies have yet to be explicitly included in performance assessment. The main features of uranium orebodies of potential relevance as an analogue are" 9
the composition,
long-term
stability and
corrosion/dissolution behaviour of uraninite as an analogue to spent fuel;
9
the role of redox processes in mobilising and retarding radionuclides, including redox fronts and other geochemical discontinuities;
9
the speciation and solubility controls of radionuclides in groundwaters, including colloid formation and behaviour; the downstream retardation processes affecting remobilised radionuclides, including sorption phenomena on various surfaces and diffusion into the rock matrix porosity; and
57
The geological disposal of radioactive wastes and natural analogues
the
ability
to
disequilibria various
use
natural
decay
series
to estimate the longevity of
mobilisation
and
deposition
The main features in clay formations of potential relevance as an analogue are: 9
the relative roles of diffusion and advection and small-scale physical heterogeneities"
9
the estimation coefficients;
processes,
3.2.2
Geochemical discontinuities in clays
Clays may be used as either a backfill or buffer in a repository, or may form the host rock itself, as discussed in Chapter 2. Geochemical transport processes in clays are not well understood, owing largely to the complexity of the multiple coupled processes of solute
of
elemental
the movement of redox fronts,
diffusion
including
movement along fractures in clays; the thermal stability of clays and thermal effects on transport properties, in cases where igneous bodies have intruded into clays; and
and clay interactions which not only drive chemical migration but also control the movement of water and the development of the hydraulic properties of the clay medium. Where geochemical discontinuities occur in clay as a result of heterogeneity in a sedimentary sequence or due to contact with a compositionally different material, such as an igneous intrusion
or
the
burial of an archaeological
artefact,
they offer the opportunity to study small scale migration and other
processes.
These processes are shown graphically in Figure 3.3.
58
Figure 3.3: Diagrammatic view of a generic clay formation showing the principal physical and chemical processes which potentially might be investigated in natural analogue studies in this type of geological environment.
Varieties of analogue studies
the chemical complexing behaviour of radionuclides and other trace elements in pore fluids of different compositions, and in contact
9
the nature and stability of colloidal species formed in high pH waters, and at the interface between high pH waters and neutral waters.
with organic material in clays.
3.2.4 3.2.3
Hyperalkaline environments
Hydrothermal systems
Natural hydrothermal systems have sometimes
Natural waters with unusually high alkalinities may be analogous to the porewaters found in
been advanced as being analogous to the nearfields of HLW or spent fuel repositories during the
cements and concretes. These offer the opportunity to study the hydrochemical behaviour
early period after disposal when temperatures around the waste are high. It has been suggested
of radionuclides and trace elements in conditions similar to those of some cementitious repository near-fields,
that they provide the opportunity to study geochemical transport processes in warm, nearfield fluids.
Hyperalkaline waters are relatively rare in nature and have their origin in uncommon systems such as complex rock/water interactions involving the alteration of ultramafic rocks (e.g. at the Oman analogue site) or thermally metamorphosed limestones and marls (e.g. at the Maqarin analogue site). To be reasonably representative of cementitious porewaters, the pH must be greater
Unfortunately, the analogy is generally weak, because repository host rock temperatures in most designs (with the possible exception of the proposed US repository at Yucca Mountain, see Section 2.3.1) generally never reach more than about 80 to 100~ which is lower than the temperature of most hydrothermal systems. In any event, most disposal concepts aim at complete containment of the wastes within the waste package during the early thermal peak. In addition, most natural hydrothermal systems have been flushed with much larger volumes of fluids than would occur around a repository, although the duration and depth of water circulation in the two systems might be comparable (Cathles and Shea, 1992).
than 12. The main features in environments of potential analogue are: 9
natural high-pH relevance as an
the solubilities and speciation of radionuclides, and other relevant elements in high pH conditions where solubility calculations suggest rates of mobilisation into solution will be very slow; the interaction of high pH fluids with surrounding rocks which may be analogous to near-field porewaters which migrate from a repository into the host rock; the nature and viability of microbially mediated geochemical processes at high pH, with respect to their potential significance for wasteform breakdown and near-field radionuclide mobilisation" and
Combined, these differences between the hydrothermal and repository systems mean that the supposed near-field rock-water interactions and mass transport analogy may be exaggerated both in terms of the scales and amounts of materials involved, and the nature and kinetics of the reactions observed. In addition, attempts to extract pertinent information for performance assessment from hydrothermally altered granites are usually made difficult by the superimposition of several hydrothermal events at various temperatures.
59
The geological disposal of radioactive wastes and natural analogues
Nonetheless, a few studies (e.g. M~nager et al., 1992a; Parneix, 1992; Shea, 1998) have been described which claim to have extracted some useful information from hydrothermal or fossil hydrothermal systems. Thus, it may be possible, although very difficult, to obtain usable information on elevated temperature near-field processes from these geological systems but the limitations of the analogy must be borne in mind, and the guideline to 'define the end-use' must be remembered. The main features in hydrothermal systems of potential relevance as an analogue are: alteration of physical properties (e.g. pore and fracture enlargement or blocking) and chemical properties (e.g. fracture and pore surface mineralogy)of near-field rocks which could occur during a thermal transient and which may affect transport and sorption of radionuclides released at a later time; the solubility and speciation of radionuclides, and relevant trace elements in geothermal fluids where hydrothermal processes have affected an existing orebody or region of rock with elevated concentrations of relevant elements; and elemental and isotopic matrix diffusion profiles associated with veins containing hydrothermally transported and deposited minerals containing relevant trace elements, from which apparent diffusivity values can be determined, These analogue applications are only really useful to high temperature repository concepts or to scenarios where early canister failure during the initial thermal peak are considered. Otherwise, these applications appear tenuous for most repository designs and normal evolution scenarios, which perhaps explains the lack of interest by performance assessment modellers in using information derived from hydrothermal systems.
60
3.2.5
Natural occurrences of repository materials
This category includes a wide range of naturally occurring materials analogous to wastes or other repository components. These are generally found as isolated and often uncommon occurrences in nature. Typical examples are glassy igneous rocks, tektites, natural bitumens, iron meteorites and native metals. These materials are appealing analogues, but frequently they are compositionally inappropriate when compared to repository materials, to an extent which may make their properties and behaviour fundamentally different. Some good analogues can, however, be found, although careful evaluation is essential if data are to be used in performance assessment. As with archaeological analogues (see below), a critical aspect is the physical and chemical nature of the environment in which they are found, and its relevance to that of a repository. The range of studies which can be envisaged is, thus, very dependent on the exact characteristics of the material and of the host system. However, natural materials also have considerable potential for illustrating the basic concepts of repository design and performance for non-technical audiences and, in this case, the demands on the analogue may be relaxed somewhat. The types of studies that have been undertaken include: 9
natural volcanic glass corrosion as an analogue for borosilicate glass wasteform dissolution" natural metal corrosion and pitting as an analogue for metal HLW and spent fuel canister corrosion; decomposition of naturally occurring bitumen as an analogue for the degradation of the bitumen L/ILW immobilisation matrix; and stability of naturally occurring cement minerals as an analogue for the long-term behaviour of cement in a L/ILW repository.
Varieties of analogue studies
3.3 Archaeological analogues The use of archaeological analogues has grown in importance over the last few years as their potential for performance assessment support and, particularly, for providing illustrations for non-technical demonstrations of safety has been more widely acknowledged (Miller and Chapman, 1995; Miller, 1996b). The progressive decay of man-made artefacts can provide a direct analogy to the long-term behaviour of repository materials and can be studied at carefully chosen archaeological sites (e.g. Figure 3.4). The types of artefacts useful as analogues ranges from jewellery to buildings. The time period of study is, naturally, constrained to a few hundreds or thousands of years at most for the majority of relevant materials. However, although shorter in comparison to geological
analogue systems, the timescales applicable to archaeological analogues may be more tightly constrained. Although the term 'archaeological' is often used, some man-made systems which are only a few decades old also yield useful information on the rates of processes relevant to the early life of repositories. In some cases, the terms anthropogenic analogue or industrial
analogue are used instead. Due to the fact that archaeological materials and the environments in which they occur can be quite dissimilar to a repository situation, it is important that care is taken when selecting archaeological systems for investigation. Not all well-preserved, old artefacts can provide quantitative analogue information for input to performance assessment. However, archaeological analogues are also very important as providers of non-technical information and as illustrations of the basic
Figure 3.4. Archaeological artefacts can be used as analogues for the long-term behaviour of engineered barrier materials. In this case, an excavation of a Roman settlement in Colchester, England provided a range of iron and bronze artefacts (rods and rings which might have been used for divination). Qualitatively, this shows the much slower corrosion of iron over bronze (copper alloy). If the chemistry of the metals and the burial environment can be shown to be similar to the repository, then quantitative corrosion rates for materials may possibly be obtained. From Crummy (1997).
61
The geological disposal of radioactive wastes and natural analogues
concepts of repository design and performance for
However, this type of investigation is potentially
non-technical audiences. This is partly due to the fact that archaeological systems can sometimes be
prone to bias if focussed on artefacts obtained from museum collections, because museums will
associated directly with materials, artefacts and time periods with which people are familiar.
(naturally) tend to house the best preserved artefacts. Corrosion rates based solely on
To aid the selection of suitable archaeological analogues for both quantitative and qualitative uses, Miller and Chapman (1995) drew-up a list of basic parameters which should be known about
archaeological
material could, thus,
conservative. This sample bias problem is likely to be less important if artefacts are collected in situ, rather than from a museum, for then it would be possible to see artefacts
the artefact, these include:
be non-
in all possible
degradation states for that environment. Analogue 9
the age of the artefact;
9
the composition of the material;
9
the deposition and preservation history; the physico-chemical environment in which the artefact was preserved;
studies based on archaeological artefacts must be considered carefully to determine if this type of bias has occurred. Nonetheless, considering all the caveats on the suitability of archaeological artefacts, the range of materials and issues for investigation is potentially wide and includes:
any events or environmental changes which have affected the artefact when in situ; and
corrosion of metallic or cementitious objects analogous to waste containers or waste matrices;
the composition of the surrounding soil or sediment. A key issue is that the burial environment in which an artefact is preserved is as important as the artefact's composition for defining its suitability as an analogue. However, characterising the burial environment can be difficult, especially determining if the environment has been constant over time. Most artefacts come from the surface or from shallow depths, often in conditions which have been more chemically aggressive than those expected in a repository, e.g. in terms of chemical fluxes and redox environment. Clearly, if the burial
degradation of glasses and cementitious or bituminous materials analogous to wasteforms; 9
long-term evolution of the physico-chemical properties of cements and other building materials analogous to repository structures;
9
decay and breakdown products of organic materials and complexation with trace elements, analogous to waste degradation; and chemical interaction of buried objects with surrounding rocks and soils which may be analogous to near-field processes.
environment has been highly chemically active, most
artefacts
will
have
been
significantly
degraded. The general approach to archaeological materials is, therefore, one of trying to estimate rates and
mechanisms
of degradation
as a
function of the chemistry of the environment of preservation, and to extrapolate or interpolate the data from one or more sites or objects to the chemical conditions expected in a repository.
62
3.4 Sites of anthropogenic contamination There are many thousands of sites around the world
where carelessly disposed wastes and
spillages
have resulted
contaminants
through
in the
migration
subsurface
of
materials.
Several of these contaminated sites have been
Varieties of analogue studies
studied as anthropogenic analogues. For example,
may occur in a repository far-field. This is
investigations have been made of:
important because, if contaminant concentrations
the migration at the geosphere-biosphere interface of radionuclides released from the Chernobyl nuclear power plant explosion; the migration
through volcanic rocks of
radionuclides released from underground nuclear explosions at the nuclear weapons test site in Nevada; and
are too high, then they may display solubility limited transport behaviour. This would be in contrast to the repository system in which many elements will be released in such dilute concentrations that they would be expected to remain below solubility limits, especially in the farfield.
the migration through rocks and sediments of radionuclides leaked from liquid radioactive waste storage tanks and shallow LLW disposal
If suitable anthropogenic sites can be found with a demonstrable analogue potential, then it may be possible to investigate migration and retardation of transuranic elements in the subsurface
trenches,
environment and, hence, provide a means to support laboratory data for these non-natural
Several other similar contamination systems could be imagined as potential analogues. For example, the injection facilities for the disposal of liquid radioactive wastes in Russia, described in Section
nuclides. Nonetheless, particular consideration and care would need to be given to the extrapolation of data from any anthropogenic
2.3, are planned to be investigated in further detail
contamination site to the repository system.
to understand how these wastes have behaved and migrated underground since injection, as an analogue of radionuclide migration away from a repository. The problem with many of these anthropogenic contamination sites is that the environments are grossly unrepresentative of either the near or farfields of any deep geological repository concept. In some cases the physical systems are greatly perturbed (such as around sites of nuclear weapons tests) or the chemistry is too far removed from repository conditions to warrant being considered as an analogue (as may be the case at the some liquid waste injection sites). As a result, the great majority of sites of anthropogenic contamination have little or no analogue relevance for a radioactive waste repository. Nonetheless, there may be a few anthropogenic contamination sites from where useful analogue information could be obtained. Of particular interest would be locations where radioactive materials
have
migrated
into
otherwise
undisturbed rocks at concentrations sufficiently dilute to be representative of the releases which
63